On Tue, Jun 21, 2016 at 10:13:45PM +0200, Arnd Bergmann wrote: > On Tuesday, June 21, 2016 5:06:25 PM CEST Catalin Marinas wrote: > > On Wed, Jun 08, 2016 at 05:49:59PM +0200, Arnd Bergmann wrote: > > > On Wednesday, June 8, 2016 1:08:29 PM CEST Catalin Marinas wrote: > > > > On Wed, Jun 08, 2016 at 03:53:46PM +0800, Jisheng Zhang wrote: > > > > > static int __init arm64_dma_init(void) > > > > > { > > > > > + if (swiotlb_force || max_pfn > (arm64_dma_phys_limit >> > > > > > PAGE_SHIFT)) > > > > > + swiotlb = 1; > > > > > + > > > > > return atomic_pool_init(); > > > > > } > > > > > > > > So any platform with RAM larger than 4GB would still initialise swiotlb. > > > > I wouldn't say it's an issue, 64MB is not a significant loss on such > > > > systems. > > > > > > > > An alternative would be to defer the freeing until we are aware of all > > > > possible dma masks for the populated devices (e.g. from DT), though I'm > > > > not sure that's enough, drivers may try to change such masks when > > > > probed. > > > > > > Right, this is a hard problem, because you can in theory have odd devices > > > that require a DMA mask lower than the limit of ZONE_DMA. > > > > I'm not sure what we can do about such devices even with swiotlb. The > > bounce buffer is allocated from ZONE_DMA and currently it assumes a > > 32-bit mask from the start of RAM, so it is not guaranteed to support a > > smaller mask. We may need to come up with some DT memory node attribute > > to define the minimum DMA-able memory and restrict ZONE_DMA during early > > boot but I would rather wait until we hit a real issue in practice. > > The bounce buffer is allocated at early boot time in order to get an address > as low as possible, in the hope that it is small enough for those cases.
The swiotlb allocation is triggered from mem_init() and that's well after the memblock has been initialised. The swiotlb_init() function uses memblock_virt_alloc_low_nopanic() which is a top-down allocator with an upper bound of ARCH_LOW_ADDRESS_LIMIT. The latter is set by the arm64 code to the top 32-bit of RAM (plus some offset), so it's likely that the swiotlb bounce buffer will be placed in the upper range of the 32-bit mask. -- Catalin